Within the field of respiratory care, for instance in the case of patient anaesthesia, it is often required to measure and monitor a great number of patient gases, such as carbon dioxide, nitrous oxide, oxygen and anaesthesia agents. This is frequently accomplished through so called lateral flow measuring analysers, which take a minor sample flow from the respiratory circuit of a patient to an adjacent instrument comprising a gas analysing unit in which the actual gas analysis takes place. The gas analysing principle is often based on the fact that many gases absorb infrared energy at different wavelengths, i.e. the wavelength absorbed is specific for the substance concerned.
Oxygen gas is however difficult to measure with this principle, since said gas exhibits no marked absorption within the same infra red range as the other gases and also the absorption peak is rather weak in comparison with said other gases. Instead the instrument often comprises a second analysing unit which measures oxygen gas separately. This second analysing unit is often based on the analysing principle which utilizes the paramagnetic properties of oxygen. This kind of unit is however expensive, rather heavy and takes up a lot of space when positioned within the analyzing instrument, where it also is difficult to access. A further drawback is that it is not possible to use a unit that performs measurements based upon the paramagnetic properties of oxygen in combination with magnetic resonance equipment, which generate strong magnetic fields.
Another possibility is to measure oxygen gas with a fuel cell. There are different kinds of fuel cells but generally they comprise an anode and a cathode separated by an electrolyte and produce electric current when supplied with reactants. The reactants are usually hydrogen gas and oxygen gas, supplied at the anode and cathode, respectively, and the electric current produced is directly proportional to the partial pressure of oxygen gas. Within this field it is important that the fuel cell measures the oxygen gas reliably and fast. For instance, a small child has a high breathing frequency, approximately 40-60 breaths per minute, which leads to that the response rise time of the fuel cell needs to be below approximately 0.5 seconds. Fuel cells with a response rise time this low are however consumed rather fast and need to be exchanged approximately once every six to twelve months.
Another issue when measuring expiration gases from a patient is that it is unavoidable that moisture, secretion, blood, bacteria etc., are liable to accompany the sample. Should these substances enter the instrument, there is a potential risk that the instrument will be permanently damaged.
WO 00/45884 discloses a liquid separator that separates liquid from gases, which comprises a water trap removably fitted in a holder unit which is connected to an analysing instrument. The water trap effectively prevents moisture and other harmful substances from entering the analysing instrument.
However, the inventors of the present application have identified a need of an arrangement for the analysis of respiratory gases, which prevents an analysing instrument of being damaged by moisture, that includes an oxygen gas measuring unit, which is less expensive, small and light weight, is easily accessible from the outside of the analysing instrument and that can be used in connection with magnetic resonance equipment.